A dynamic light scattering method and a microscopic observation method have been known as methods for measuring particle sizes of particulates. The dynamic light scattering method is used for measuring the particle sizes in nanoscales. The dynamic light scattering method measures the particle sizes through data analysis of intensity of scattered light obtained through laser light irradiation. The data analysis is performed based on the assumption that particles sizes are in normal distribution. However, it is unclear whether or not actual particle size distribution is in normal distribution. The microscopic observation method measures the particle sizes through image analysis performed on an individual particle basis. However, the microscopic observation method requires lots of efforts for the analysis.
To address a problem as described above, a particle size measurement method using a light interference phenomenon has been developed (see Non-Patent Literature 1). Non-Patent Literature 1 discloses a particle size measurement device used for particle size measurement of particulates. The particle size measurement device has two glass plates. The two glass plates are attached to each other at respective one ends thereof, thereby forming a wedge-shaped gap between the two glass plates. Distance between the two glass plates increases with an increase in distance from a position where the two glass plates are attached to each other at the respective one-ends thereof. Upon light beam irradiation to the particle size measurement device, an interference fringe repeatedly having a bright line and a dark line is caused to appear due to an interference phenomenon. Observation of the number of dark lines in the interference fringe provides the distance between the two glass plates. In particle size measurement, particulates targeted for the particle size measurement are delivered into the wedge-shaped gap. Each of the delivered particulates is trapped at a portion of the gap in accordance with a size of the particulate. A stop position of the particulate can be measured to obtain the particle size thereof.